I'm recently installed Kubuntu 15.10 (KDE 5.4.2, kernel 4.2.0-16-generic). I was using a dual monitor set up in Kubuntu 15.04. Suddenly the dual monitor set up does not work.

If I connect the monitor (HDMI) then my laptop screen goes dark and I can see the mouse in the left edge of the external monitor. I can move it up and down the edge, but the monitor and my laptop are both dark. I cannot move the mouse into my laptop or my monitor. Nothing else shows. Once I disconnect the monitor, KDE seems to crash. Effects do not work correctly, and often windows stop accepting keyboard input (though some shortcuts and the mouse will still work).

Any ideas? I can Ctrl + Alt + F2 into a tty terminal and log in, so if there's any troubleshooting I could do from there I'd be happy to try as well. Many thanks!

Without having seen a backtrace, I bet you're running into https://bugs.kde.org/show_bug.cgi?id=341497 - which is a severe Qt5 bug.
Basically every Qt5 bug can easily crash whenever you touch the screen layout. It's to be improved (but afaics not really fixed) in Qt 5.6 ...

This seems to have fixed itself after a few rounds of updates. Oddness, but I'm glad it's fixed.

Aaaand I spoke too soon! I hadn't rebooted after the latest updates. After reboot the problem is back. So odd! I'll hope some later update round reinstates the fix that appeared earlier.

This does not fix it self. It's a major and still present bug in Qt5.
Despite being claimed fixed a dozen times, that everytime just exposed a new nullptr deref. It will take time until that's fully gone (if ever; and they refused to do the obvious and add an invalid dummy screen)

Wow. That is very hard to hear. Thanks for being upfront about it, though.

I know I don't answer the question, but I just want to mention that for what concerns external monitors, for me it's much better
after upgrading to kubuntu 15.10. In 15.04 it used to be *very* slow (when I plugged in the monitor, I could easily wait 30 sec till the image stabilizes, and sometimes it would freeze for 5 to 10 minutes...). Now it's almost immediate !
(yes, if I play too much with the settings, it can crash, but that's ok. so far I am much more satisfied)

I came here to report the same issue. Problem is exactly as described.

• With external monitor either plugged or unplugged, login to plasma session.
• Both external and primary monitors go black. Mouse is visible on the left hand side of the external monitor but can only move vertically.

• With external monitor unplugged, login to plasma session.
• Once logged in, plug in external monitor.
• Both external and primary monitors go black. Mouse is visible on the left hand side of the external monitor but can only move vertically.

Ticket referenced by @luebking is marked as resolved upstream, yet this issue persists. In any case, I'm unconvinced that this is the cause.

"Resolved upstream" does *not* imply "fixed" but say "someone elses problem"
To be sure it's the bug, inspect the backtrace of the plasmashell crash, if it says something about QScreen and 0x0, you hit one of the *many* instances of this bug.

Thank you @luebking for the correction. Are you aware of any potential workarounds for this issue? I use KDE on my work machine, and I can't continue to operate without an external monitor. If this issue persists I'll need to switch distributions.

There's nothing such as a workaround for a nullpointer dereferenciation, disabling the kscreen daemon in "kcmshell5 kded" and configuring the displays via xrandr should mitigate the problem (yet not avoid it for sure)

You basically need to avoid randr events (what's hardly possible, notably not with > 1 screens) or Qt5 or downgrade Qt5 to Qt5.3 (implies downgrading KDE, probably no good idea)

Distro hopping won't help you.
The "ooops count" should have dropped in latter 5.5 versions and upcoming 5.6, but there still seem open bugs.

The alternative is to use KDE4 or any other software that does not build upon Qt5

Just to be sure that it is this bug: you get a plasmashell crash dialog resp. no plasmashell process is running anymore? (Not that this is simply some new & blank activity)

Hi

in fact I too am having quite a lot of instabilities with Kubuntu 15.10 when plugin external monitors or beamers.
This is also my work laptop, but I use external monitors every day, and because I also had other
serious troubles, I have now decided to switch to Mint KDE, which is still using kde 4.

I'll report how it goes

This will hopefully be resolved with Qt5.6 (please notice that it's not related to the session you run - *every* application that links Qt5 is prone to crash on such occasions, ie. it will affect a Qt5 qupzilla under KDE4 just as much. The special KDE5 threat is a crash in ksmserver because of this, as it'll hard-exit your session)

Just to add information.
Have a dual monitor setup with Nvidia graphics board. Running Ubuntu 15.10. KDE Plasma 5 (I think). Been working with Python and PyQt5.

Windows would lose the decorator with exit, minimize, etc buttons. Restarting kwin_x11 would fix the problem, but it came back.

This is an incomplete test, but I find that as long as I don't move a window from one monitor to the other, kwin_x11 does not crash and have not had problem with loss of the decorator bar.